The present invention relates to an arrangement for the automatic control of the operation of the sheet-feeding unit and sheet-presenting unit of a printing machine in dependence upon the position of sheets in the sheet-presenting unit as determined by sensing devices.
In the operation of a printing machine of the type in question, the sheets to be printed are fed to the machine by a sheet-feeding unit. The feeding unit picks the sheets to be printed off of a stack of such sheets and feeds them to the actual sheet-presenting unit of the printing machine proper via, for example, an inclined feed table provided with conveyor belts. Conventionally, the transport of sheets is initiated by means of an activator element, such as a start button. The monitoring of the sheets in the sheet-presenting unit of the printing machine proper is performed by means of sensing devices. When the sensing devices detect the presence in the sheet-presenting unit of a properly positioned sheet (hereinafter referred to as a good sheet), the sheet-presenting unit is enabled for activation. When the sensing devices detect the presence in the sheet-presenting unit of no sheet (hereinafter referred to as a missing sheet), an improperly positioned sheet (hereinafter referred to as a skew sheet), or a plurality of superimposed sheets stuck together (hereinafter referred to as a multiple sheet), the sheet-feeding unit is automatically shut off, and the sheet-presenting unit of the printing machine proper is shut off or disabled so that it cannot be activated; this is disclosed for example in German Democratic Republic Pat. No. DL 68,691, the entire disclosure of which is incorporated herein by reference. Hereinafter, the expression bad sheet is employed to refer, generically, to a missing sheet, a skew sheet, or a multiple sheet.
A disadvantage of these known automatic control systems is that, when the operator presses the start button for the sheet-feeding unit, the sheet-feeding unit commences to operate, irrespective of whether the sheet present in the sheet-presenting unit of the printing machine proper is a good sheet, or one of the aforementioned three types of bad sheet, i.e. irrespective of whether the proper conditions for start-up actually exist.
Another disadvantage of these known automatic control systems is that, after the start-up of the sheet-feeding unit and sheet-presenting unit, if a bad sheet is detected as being present in the sheet-presenting unit, the sheet-feeding unit is always shut off in automatic response. This occurs irrespective of the fact that, the detected bad sheet often becomes converted into a good sheet. For example, if the bad sheet is a skew sheet, the leading end of the next sheet often straightens out the skew sheet, converting it into a good sheet.